Hes not one to let things go (6)
I believe the answer is:
keeper
'go' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for keeper that I've seen before include "Custodian; bar across magnet" , "penalties will come my way" , "Cain asked 'Am I my brother's ......?'" , "Zoo attendant" , "Zoo worker" .)